1. Field of the Invention
This invention relates to a method for dispensing a catalyst solution into a coating formulation as the formulation is applied to a surface using a hand-held liquid spraying apparatus.
2. Description of the Prior Art
Automobile refinish clearcoats typically comprise a three-dimensional cross-linked polymer formed by two major reactive components. One component comprises polyol oligimers with multiple hydroxl end groups. The other component comprises organic molecules having isocyanate functional groups, such as a trimer of hexamethylene di-isocyanate. This hydroxyl-isocyanate chemistry is also employed for certain primers as well as for monocoats.
These two components are generally packaged as separate formulations in a volatile solvent, such as ethyl acetate, and are sold in separate containers.
At least one of the component formulations, usually the one having the hydroxyl oligomers, also contains a polymerization catalyst, such as dibutyl tin di-laurate, DBTBL. The catalyst promotes the rate of polymerization when the two components are mixed. The volatile solvent reduces viscosity for effective spraying. The formulations may also include relatively minor amounts of additives such as viscosity modifiers and/or retarders of catalytic activity.
In the spraying technology practiced currently in refinish shops the two component formulations are mixed prior to spraying and placed in a cup-like reservoir that is attached to a hand-held spraying apparatus. Due to the presence of catalyst, polymerization begins as soon as the component formulations are mixed. Thus, the viscosity of the mixture increases both before and while it is being sprayed.
The time it takes for the viscosity to increase to a point where spraying becomes ineffective, generally a two-fold increase in about thirty (30) minutes, is termed “pot life”. There is available only a relatively short time window before the mixture becomes unusable. The possibility that the spray gun itself may become clogged with cured material is also disadvantageous.
One way to extend pot life is to add a greater amount of thinning solvent to the mixture. However, thinning agents contribute to increased emissions of volatile organic compounds and also increase the curing time. Thus, this alternative is not particularly attractive.
Other prior art attempts to extend pot life of the coating formulation have focused on “chemical-based” solutions.
For example, it has been suggested to include in the component formulation(s) certain additives that would retard polymerization in the mixing pot. However, the additives must be such that the rate of curing is not adversely affected after the coating is applied to the surface.
These chemical-based solutions may increase pot life to some degree. For example, clearcoats sold by E. I. Du Pont de Nemours and Company have a pot life of about one (1) to two (2) hours. Another suggested alternative is to include relatively inactive catalysts which becomes active form upon exposure to air after atomization.
Accordingly, in view of the foregoing it is believed advantageous to extend pot life of the coating formulation [on the order of four (4) to seven (7) hours] in a way that does not increase volatile organics in the formulation and does not delay the curing of the applied coating.